This invention generally relates to a fuel control apparatus for an engine which is primarily used for a vehicle which permits the changing of a valve opening-closing timing and/or a valve-lift amount for a valve on at least one of an intake side and an exhaust side. This change of the valve opening-closing timing and/or the valve-lift amount will be generally referred to as the "change of valve timing".
An engine wherein the valve timing of the intake and exhaust valves can be switched over to a low-speed valve timing whereby a valve-open duration and a valve-lift amount are small and suitable for a low-speed revolution range and for a high speed valve timing where the valve-open duration and the valve-lift amount are large, and further suitable for a high-speed revolution range is known in Japanese Examined Patent Application Publication No. 33289/1974.
Japanese Unexamined Patent Application Publication No. 191426/1982 discloses a method having a no-load reference value which substantially conforms to a P.sub.B characteristic representing an intake negative pressure P.sub.B at each revolution speed of the engine when the engine with its transmission set in neutral is operated in such a manner that its revolution speed may be kept unchanged.
The method disclosed in the above-described publication further has a pressure sensor which detects an intake negative pressure employed as a load detecting means to detect the engine load, and wherein fuel supply to the engine is stopped in a range where the negative pressure detected is below the no-load reference value (i.e., closer to vacuum). Referred to as the no-load P.sub.B characteristic is an intake negative pressure necessary for generating an output that can make up for the friction loss of the engine. When this intake negative pressure is lower than the no-load characteristic, the engine performs a negative work in which the engine may be rotated by a counter driving torque from the driving wheel side so that the range wherein the negative pressure is lower than the no-load reference value is taken as a deceleration operation range and fuel supply to the engine is thus stopped.
In an engine permitting to change the valve timing, the no-load P.sub.B characteristic also changes with the change of the valve timing and is represented by line a at a low-speed valve timing and by line b at a high-speed valve timing shown in FIG. 3. A revolution speed range where the no-load P.sub.B is relatively low is different between an operation with the low-speed valve timing and an operation with the high-speed valve timing. Accordingly, if a predetermined operation range where the fuel supply is to be stopped is set so as to match with one of the just-described valve timings, the fuel supply to the engine is stopped in a relatively high load range when the engine is operating at the other valve timing, and the engine drivability thus deteriorates.
In the state of operation where the intake negative pressure is slightly higher than the no-load P.sub.B, (i.e., when a vehicle is cruising at a low-load and at a constant speed) it is desirable to set the air-fuel ratio of the mixture somewhat to a leaner side than a theoretical air-fuel ratio for improving fuel economy. To this end, one alternative is a structural arrangement whereby a low-load reference value is set which is slightly higher than the no-load P.sub.B characteristic, and the fuel supply quantity to the engine is decreased in a low-load operation range where the intake negative pressure falls between the no-load reference value, as described above, and the low-load reference value. However, if this low-load operation range is set in conformity with one of the valve timings, the same above-described problem develops.